The present invention relates to a base station, a communication terminal, and a wireless communication system in which the base station and the communication terminal transmits and receives data through a wireless network, and, more particularly, to an interference control technique for a base station including plural antennas.
In general, in a digital mobile communication system using OFDMA (Orthogonal Frequency Division Multiple Access), as disclosed in U.S. Patent Application Publication US2007/0207838, communication is made between plural terminals in a certain frequency bandwidth and time unit (called a frame), in more particular, using wireless resources divided in frequencies and time. In IMT-Advanced as the fourth generation communication system using the OFDMA, for improvement in the throughput, both a multicarrier technique and a MIMO (Multiple-Input Multiple-output) technique are applied, as disclosed in 3GPP, TR 36.814 v9.0.0, March 2010 (see Multicarrier p 8 Section 5).
In the multicarrier technique, a base station can use plural frequency bands at the same time. For example, in a system configured with three frequency bands with a bandwidth of 10 MHz, each base station uses the entire three frequency bands of 10 MHz for communication with the terminals, thereby improving the throughput. Generally, in the OFDMA system, the interference occurs between cells if the same frequency is used between neighbor cells, thus degrading the throughput. In the IMT-Advanced in which each base station can use plural frequency bands, the same frequency band is used between the neighbor cells, thus requiring a technique for minimizing the inter-cell interference. As a method for minimizing the inter-cell interference, the beamforming technique is known as one for restricting antenna beams in a certain range. For example, in a terminal, the received signal power is high, when the beams from its own base station are directed to the terminal. On the contrary, the interference power is high, when the beams from a neighbor base station using the same frequency are directed to the corresponding terminal. An article, IEEE802.16, P802.16 m/D9, October 2010, (See Multicarrier, p 537 FIG. 496), introduces a technique. According to this technique, a base station has a beam pattern that is changed to a frequency direction, different combinations of the frequency and the beam pattern are set between the base station, thereby forming at least one frequency with less interference for each terminal, and wireless resources are assigned to the frequency, thereby minimizing the interference.
On the other hand, in the MIMO technique, to improve the throughput, plural signals are multiplexed into the same wireless resource using plural transmitter/receiver antennas. In the base station, the user throughput and the cell throughput are improved by adaptively combining two techniques. The techniques are SU (Single-User)-MIMO for multiplexing plural signals for one terminal and MU (Multi-User)-MIMO for multiplexing signals for plural terminals.